1. Field of the Invention
The present invention relates to an engine control method, in particular, to a method for judging engine combustion condition of cylinders based on a crank angle speed detected in a lean burn condition and the like, and for controlling the engine and to a control system based on the judgment of the combustion condition.
2. Related Art
Conventionally, a method and system for detecting a crank angle speed of an engine crank shaft, for judging the combustion condition of each of the cylinders based on variations thereof and for controlling the engine based on the judgment of the combustion conditions have been generally known. For example, Japanese Patent Un-examined Publication No. 3-15645 discloses a multiple cylinder engine control system provided with means for detecting the crank angle speed when each of pistons of the cylinders are positioned at the same stroke points respectively, obtaining crank angle speed data based on the detection, judging the combustion conditions of each of the cylinders and controlling a fuel injection amount and ignition timing and the like.
In the above system, if the crank angle speed data can be obtained with the same condition for the respective cylinders and the crank angle speed is detected based on the measurement of a relatively short term (about 20.degree. crank angle), a crank angle speed variation can be readily detected even in an idle operation and low speed operation.
It should however be noted that although the respective crank angle detections are expected under the same conditions, the combustion condition of other cylinders would influence the measurement of the crank angle speed of the object cylinder depending on the selection of the portion of the crank angle for measurement so that a correlation between the combustion conditions and the crank angle speeds of each of the cylinders. Thus, the system disclosed in the above publication is disadvantageous that the judgment of the combustion conditions of the engine could not be properly detected based on the crank angle speed detection. There is a need for a system for judging combustion conditions of the cylinders properly.
In another aspect, there has been known an engine making a lean-burn operation in which the engine is operated with a leaner intake gas mixture of an air fuel ratio (also referred to as A/F) having a larger value than a theoretical value (14.7). In an engine making the lean-burn operation in an off-idling operating range (operating range other than an idling operating range), it is desirable that the engine combustion condition is accurately judged based on the measurement of the crank angle speed as to whether an combustion is properly being done under such a lean-burn operation. In particular, it is desirable that the combustion condition is properly judged based on the crank angle detection even in a relatively high speed and high load condition of the lean-burn operation range. In this respect, it would be difficult for the above system as disclosed in the Japanese publication 3-15645 to detect the crank angle speed with a satisfactory precision in the lean operation range due to a shorter range for crank angle measurement, and thus, due to a shorter time period for the measurement, particularly in a middle and high engine speed condition.